The present invention relates to a mass spectrometer, and particularly to a liquid chromatograph/mass spectrometer in which a liquid chromatograph is coupled with an ion trap type mass spectrometer.
Recently, in the field of analysis, it is required to establish a technique of analyzing a mixture. For example, in the case of analyzing harmful substances in environments, a sample taken for analysis (for example, water in lakes and marshes) contains a variety of substances. The same is true for analysis of substances associated with organisms. A sample derived from an organism, such as blood or urine, contains a variety of substances. In this way, the technique of analyzing a mixture is essential to analysis of substances associated with environments and substances associated with organisms.
In general, it is difficult to directly analyze a mixture. Accordingly, a mixture is separated into components, each of which is in turn detected and identified. In such circumstances, a liquid chromatograph/mass spectrometer (hereinafter, referred to as xe2x80x9cLC/MSxe2x80x9d) and a capillary electrophoresis/mass spectrometer (hereinafter, referred to as xe2x80x9cCE/MSxe2x80x9d) in which a liquid chromatograph and a capillary electrophoresis good in separation are respectively coupled with a mass spectrometer good in identification of a substance are very useful for analysis of the above-described substances associated with environments and organisms.
A prior art LC/MS using a mass spectrometer having an ion trap type mass spectrometric unit will be described with reference to FIG. 14.
A liquid chromatograph 1 includes a liquid supply pump 2, a mobile phase solvent bath 3, a sample injector 4, a separation column 5, and a pipe 6. The mobile phase solvent is supplied at a specific flow rate from the liquid supply pump 2 to the separation column 5. A mixture sample is introduced from the sample injector 4 disposed between the liquid supply pump 2 and the separation column 5. The sample, which has reached the separation column 5, is separated into components by interaction with a filler charged in the separation column 5. The sample, whose components have been separated by the liquid chromatograph 1, is introduced together with the mobile phase solvent into an ion source 7.
Of known various type of ion sources, a typical electrostatic spraying type will be described below. The sample, which has reached the ion source 7, is introduced in a metal tube 9a via a connector 8. When a high voltage of several kilovolts is applied from a high voltage power supply 11 between the metal tube 9a and an electrode 10 disposed opposite to the metal tube 9a, electrostatic spray is generated in the direction of the counter electrode 10 from the end of the metal tube 9a. The flow rate of a solution allowing to sustain stable electrostatic spraying is about several microliters per minute; however, the flow rate of the solution supplied from the liquid chromatograph 1 to the ion source 7 is about one milliliter per minute, and accordingly, a spraying gas 13 supplied from a gas supply pipe 12 is allowed to flow around the metal tube 9a to assist electrostatic spraying with the gas 13. Droplets created by electrostatic spraying, which contain ions associated with sample molecules, are dried into gaseous ions. The ions thus created are introduced in a vacuum unit 17 pumped by a pumping system 15b via an ion introducing pore 14a opened in the counter electrode 10, a differential pumping portion 16 pumped by a pumping system 15a, and an ion introducing pore 14b. An electrostatic lens 19a composed of electrodes 18a and 18b is disposed in the differential pumping portion 16, which lens acts to converge ions for improving the permeability of the ions through the pore 14b. The ions introduced in the vacuum unit 17 are converged through a converging lens 19b composed of electrodes 18c, 18d and 18e, and then introduced in an ion trap mass spectrometric unit 20.
Next, the operational principle of the ion trap mass spectrometric unit will be described. The ion trap mass spectrometric unit 20 includes a ring electrode 21 and end cap electrodes 22a and 22b. FIG. 15 is a diagram showing a control of the amplitude of a high-frequency voltage applied to the ring electrode with an elapsed time in a period of time required for obtaining a first mass spectrum (the change in voltage applied to an electrode with an elapsed time, as shown in the figure, is hereinafter referred to as xe2x80x9cscan functionxe2x80x9d). First, in an ion storage period. 201, a high-frequency voltage is applied to the ring electrode 21 to form a potential for confinement of ions in a space surrounded by the ring electrode 21 and end caps electrodes 22a and 22b. The ions trapped in the vacuum unit 17 are converged through the converging lens 19b to enter into the space surrounded by the ring electrode 21 and the end cap electrodes 22a and 22b from an opening 23a formed in the end cap electrode 22a. An impingement gas such as helium is introduced in the space surrounded by the ring electrode 21 and the end cap electrodes 22a and 22b, and is kept at a pressure of about 1 milli-Torr. The ions impinge on molecules of the impingement gas to lose the energies thereof, and are confined in the confinement potential formed in the space surrounded by the ring electrode 21 and the end cap electrodes 22a and 22b. Next, in a scan period 202, a voltage applied to either of the electrodes 18c, 18d and 18e constituting the converging lens 19b is changed, to prohibit the ions from passing through the converging lens 19b, thereby preventing entrance of the ions into the ion trap mass spectrometric unit 20. The mass analysis is performed by gradually increasing the amplitude of the high-frequency voltage applied to the ring electrode 21. It is known from a document xe2x80x9cPractical Aspects of Ion Trap Mass Spectrometry, vol. 2, p. 10(CRC Press, 1995)xe2x80x9d that in the ion trap mass spectrometric unit, the ion trajectory becomes unstable in the direction of the end cap electrode (the direction of the Zo axis shown in FIG. 14) if a q value defined in the following equation is more than 0.908.
q=8zV/m(ro2+2Zo2)xcexa92xe2x80x83xe2x80x83(Equation 1)
In this equation, z designates the electric charge of an ion; V is the amplitude of a high-frequency voltage applied to the ring electrode; m is the mass of the ion; r0 and Z0 are a radius of the circle inscribed with the ring electrode 21 and the distance from the center of the circle to each of the end cap electrodes 22a and 22b respectively; and xcexa9 is an angular frequency of the high-frequency voltage applied to the ring electrode 21. Accordingly, in the scan period 202, as the amplitude V of the high-frequency voltage applied to the ring electrode 21 is gradually increased, the trajectories of the ions become unstable sequentially in the order from an ion having a smaller value obtained by dividing the mass of the ion by the electric charge of the ion (hereinafter, referred to as xe2x80x9cm/zxe2x80x9d) to an ion having a larger value of m/z, and the ions are sequentially discharged from openings 23a and 23b formed in the end cap electrodes 22a and 22b to the outside of the mass spectrometric unit 20. The discharged ions are detected by an ion detector 24, and detection signals are supplied to a data processor 26 via a signal line 25, to be thus processed. After termination of the scan period 202, the voltage applied to the ring electrode 21 is cut off, to destroy the ion confinement potential, thereby removing the ions remaining in the mass spectrometric unit 20 (ion removing period 203). These sequences of operations (ion storage period 201, scan period 202, and remaining ion removing period 203) are repeated, to perform mass analysis of the samples sequentially supplied from the liquid chromatograph 1.
While not shown in FIG. 14, the liquid chromatograph 1, ion source 7, electrostatic lenses 19a and 19b, and ion trap mass spectrometric unit 20 are controlled by a control unit (including a controlling power supply, control circuit, and control software).
The above-described prior art is disclosed in a document xe2x80x9cAnalytical Chemistry, vol. 63, p. 375, 1991xe2x80x9d, and the operational principle of the ion trap mass/spectrometric unit is disclosed in U.S. Pat. No. 4,540,884.
The above-described prior art has the following problems:
In the ion storage period 201, a high-frequency voltage having a specific amplitude is applied to the ring electrode 21, and accordingly, as is apparent from Equation 1, the q values of ions having different values of m/z are different from each other. It is known that when ions created in a source outside the ion trap mass spectrometric unit 20 are allowed to enter in the mass spectrometric unit 20, the confinement efficiency of the ions from the outside in the ion trap mass spectrometric unit 20 is dependent on the q values of the ions. In accordance with the description in a document xe2x80x9cPractical Aspects of Ion Trap Mass Spectrometry, vol. 2, p. 75 (CRC Press, 1995)xe2x80x9d, an ion having a q value ranging from about 0.4 to 0.5 can be efficiently confined in the ion trap mass spectrometric unit 20; however, the confinement efficiency of an ion having a q value out of the above range is poor. In the mass spectrometer having an ion trap mass spectrometric unit 20, since ions confined in the mass spectrometric unit in the ion storage period 201 are discharged outside the mass spectrometric unit 20 in the scan period 202 to be detected, there is a close relationship between the confinement efficiency of the ions and the detection sensitivity of the ions. As a result, in the LC/MS having the prior art ion trap mass spectrometric unit, ions different in the q value (that is, different in m/z) are different in confinement efficiency in the ion trap mass spectrometric unit 20, they become different in detection sensitivity. In other words, if the q value is optimized (as is apparent from Equation 1, this means that the amplitude of a high-frequency voltage in the ion storage period 201 is optimized) for an ion having a certain value of m/z, there arises a problem that the above ion is efficiently confined in the ion trap mass spectrometric unit 20 and thereby it can be detected with a high sensitivity; however, another ion having a value of m/z different from the above one corresponding to the optimized q value is not efficiently confined in the ion trap mass spectrometric unit 20 and thereby it cannot be detected with a high sensitivity.
FIG. 16 shows a change in mass spectrum depending on the amplitude of a high-frequency voltage in the ion storage period 201, in a test using a mass spectrometer having the prior art ion trap mass spectrometric unit. In this test, the sample was prepared by dissolving two kinds of polyethylene glycol (structural formula: HOxe2x80x94(CH2xe2x80x94CH2xe2x80x94O)nxe2x80x94H) having average molecular weights of 200 and 600 in water at each concentration of 10 xcexcmol/l. When the amplitude of the high-frequency voltage upon ion storage was set at 150 V, the cluster ion (H3O+(H2O), m/z=37) of protonated water used for a solvent was strongly observed; however, in a range of relatively large values of m/z (m/z greater than 300), ions were little observed. On the other hand, when the amplitude was set at 460 V, the intensity of the cluster ion (m/z=37) of water became lower, and even in a range of m/z greater than 500, ions of protonated molecules of polyethylene glycol were observed with high sensitivities. FIG. 17 is a graph showing the result of examining the relationship between the amplitude of the high-frequency voltage in the ion storage period 201 and the ion intensity, using typical values selected from the peaks of the above data of polyethylene glycol shown in FIG. 16. The ion (m/z=195) was most strongly observed at the amplitude of 400 V; however, at this amplitude, the intensity of the ion (m/z=723) was weak. On the other hand, the intensity of the ion (m/z=723) was most strongly observed at the amplitude of 585 V; however, at this amplitude, the intensity of the ion (m/z=195) was reduce to about half the maximum value. In this way, if the amplitude of the high-frequency voltage in the ion storage period 201 is kept constant, the range of the values of m/z of ions detectable with high sensitivities is narrow, thereby making it difficult to analyze ions in a wide range of values of m/z with high sensitivities.
If a substance to be analyzed is known, the values of m/z of ions derived from the substance can be estimated, and accordingly, the amplitude of a high-frequency voltage in the ion storage period 201 can be previously set at a value allowing the ions to be detected with high sensitivities. In the case where the values of m/z of ions cannot be estimated, however, the amplitude must be roughly set, so that the ions of the sample cannot be necessarily detected with high sensitivities. This causes a large problem particularly in the case of automatic analysis of an unknown sample, significantly degrading the reliability of the mass spectrometer.
In view of the foregoing, it has been expected to develop a mass spectrometer capable of detecting ions in a wide range of values of m/z at high sensitivities.
An object of the present invention is to provide a mass spectrometer having an ion trap type mass spectrometric unit capable of obtaining a mass spectrum in a wide range of values of m/z of ions, while not giving any laborious work to an operator in setting the amplitude of a high-frequency voltage in an ion storage period, by superimposing a plurality of mass spectra obtained under different ion storage conditions (different amplitudes of the high-frequency voltage applied to a ring electrode in the ion storage periods) and outputting the superimposed spectra as one mass spectrum.
The above object can be achieved, according to the present invention, by provision of a mass spectrometer including: an ion source for ionizing a sample; an ion introducing pore for trapping ions created in the ion source into a vacuum unit; and an ion trap mass spectrometric unit disposed in the vacuum unit; wherein the inside of the ion trap mass spectrometric unit has the setting of ion storage periods in each of which the ions are stored in the ion trap mass spectrometric unit, and mass scan periods in each of which the ions stored in the ion trap mass spectrometric unit are discharged outside the ion trap mass spectrometric unit depending on values of the ions, each value being obtained by dividing the molecular weight of the ion by the valence number of the ion and the mass spectrum of the ions thus discharged are detected; the mass spectrometer being characterized in that the amplitude of a high-frequency voltage applied to a ring electrode constituting part of the ion trap mass spectrometric unit in each of the ion storage periods is set at different values before and after an arbitrary one of the mass scan periods. The above object can be also achieved by provision of mass spectrometer including: an ion source for ionizing a sample; an ion introducing pore for trapping ions created in the ion source into a vacuum unit; and an ion trap mass spectrometric unit disposed in the vacuum unit; wherein the inside of the ion trap mass spectrometric unit has the setting of ion storage periods in each of which the ions are stored in the ion trap mass spectrometric unit, and mass scan periods in each of which the ions stored in the ion trap mass spectrometric unit are discharged outside the ion trap mass spectrometric unit depending on values of the ions, each value being obtained by dividing the molecular weight of the ion by the valence number of the ion and the mass spectrum of the ions thus discharged are detected; the mass spectrometer being characterized in that the amplitude of a high-frequency voltage applied to a ring electrode constituting part of the ion trap mass spectrometric unit in each of the ion storage periods is changed within each of the ion storage periods. The above object can be also achieved by provision of a mass spectrometer including: an ion source for ionizing a sample; an ion introducing pore for trapping ions created in the ion source into a vacuum unit; and an ion trap mass spectrometric unit disposed in the vacuum unit; wherein the inside of the ion trap mass spectrometric unit has the setting of ion storage periods in each of which the ions are stored in the ion trap mass spectrometric unit, and mass scan periods in each of which the ions stored in the ion trap mass spectrometric unit are discharged outside the ion trap mass spectrometric unit depending on values of the ions, each value being obtained by dividing the molecular weight of the ion by the valence number of the ion and the mass spectrum of the ions thus discharged are detected; the mass spectrometer being characterized in that the amplitude of a high-frequency voltage applied to a ring electrode constituting part of the ion trap mass spectrometric unit in each of the ion storage periods is set on the basis of information obtained by a mass spectrum which has been previously obtained at an arbitrarily set amplitude. The above object can be also achieved by provision of a mass spectrometer including: an ion source for ionizing a sample; an ion introducing pore for trapping ions created in the ion source into a vacuum unit; and an ion trap mass spectrometric unit disposed in the vacuum unit; wherein the inside of the ion trap mass spectrometric unit has the setting of ion storage periods in each of which the ions are stored in the ion trap mass spectrometric unit, and mass scan periods in each of which the ions stored in the ion trap mass spectrometric unit are discharged outside the ion trap mass spectrometric unit depending on values of the ions, each value being obtained by dividing the molecular weight of the ion by the valence number of the ion and the mass spectrum of the ions thus discharged are detected; the mass spectrometer being characterized in that portions, equivalent to arbitrary values of m/z (molecular weight of ion/valence number of ion), of a plurality of mass spectra obtained by changing the amplitude of a high-frequency voltage applied to a ring electrode constituting part of the ion trap mass spectrometric unit in each of the ion storage periods are coupled with each other, and are outputted as one mass spectrum. The above object can be also achieved by provision of a mass spectrometer including: an ion source for ionizing a sample; an ion introducing pore for trapping ions created in the ion source into a vacuum unit; and an ion trap mass spectrometric unit disposed in the vacuum unit; wherein the inside of the ion trap mass spectrometric unit has the setting of ion storage periods in each of which the ions are stored in the ion trap mass spectrometric unit, and mass scan periods in each of which the ions stored in the ion trap mass spectrometric unit are discharged outside the ion trap mass spectrometric unit depending on values of the ions, each value being obtained by dividing the molecular weight of the ion by the valence number of the ion and the mass spectrum of the ions thus discharged are detected; the mass spectrometer being characterized in that the amplitude of a high-frequency voltage applied to a ring electrode constituting part of the ion trap mass spectrometric unit in each of the ion storage periods is set depending on a substance to be analyzed.